1. Field of the Invention
The present invention relates to the synthesis of ceramic structures and more particularly, to the synthesis of ceramic structures of barium aluminum silicates (BAS). Still more particularly, this invention relates to the formation of monoclinic celsian (BaO.Al.sub.2 O.sub.3.2SiO.sub.2) using a fluorine catalyzed sol-gel process. Additionally, the present invention relates to the synthesis of monoclinic SrO.Al.sub.2 O.sub.3.2SiO.sub.2 (SAS) structures.
2. Description of the Related Art
Monoclinic celsian (BaO.Al.sub.2 O.sub.3.2SiO.sub.2) materials are known for a combination of high melting point, low thermal expansion, high thermal shock resistance, low and thermally stable dielectric thermal expansion of 2.29.times.10.sup.-6 /.degree. C. (at 20.degree. C. to 1000.degree. C.), bending strength up to 110 MPa, and dielectric constant of 6-7.times.10.sup.-4 and loss tangent of 1-2.times.10.sup.-4 at 600.degree. C. These ceramics may be prepared from natural materials, such as kaolin or clay, or technical grade starting materials containing significant amounts of impurities. However, impurities can adversely affect all properties, particularly the dielectric behavior of ceramics, especially at high temperatures.
The polymorphism of celsian is complex. Generally, monoclinic celsian is stable at temperatures less than 1590.degree. C., whereas hexacelsian is stable at temperatures from 1590.degree. C. to the melting temperature of 1760.degree. C. Although the hexagonal structure is stable at temperatures above 1590.degree. C., it tends to be the first product of solid phase reaction and has a strong tendency to persist metastably over the entire temperature range. Hexagonal celsian transforms reversibly into a low temperature orthorhombic form at 300.degree. C. This transformation is accompanied by significant volume changes of approximately 3% to 4%, which significantly hinders most practical uses of hexagonal celsian in high temperature ceramic materials, especially thermal cycling applications.
Transformation of hexagonal celsian into monoclinic celsian is promoted by prolonged high temperature heating at greater than 1450.degree. C., hydrothermal treatment at about 2 Kbar pressure, formation of glass phase during firing, and by the presence of impurities or the addition of certain additives, such as B.sub.2 O.sub.3, LiF, Cr.sub.2 O.sub.3, CaF.sub.2, ZrSiO.sub.3.
Monoclinic SrO.Al.sub.2 O.sub.3.2SiO.sub.2 (SAS), strontium aluminosilicate, has high refractoriness, low thermal expansion, low dielectric constant and loss tangent both stable over a broad range of temperatures and frequencies. SAS has a melting point of 1710.degree. C. and exhibits similar polymorphism as BAS. SAS formation by solid phase reaction also is characterized by the primary appearance of a metastable hexagonal form, which is highly unstable in relation to BAS. Accordingly, preparation of monoclinic SAS does not present the problems encountered with the preparation of monoclinic BAS. A minimum temperature of 1550.degree. C. has been needed to produce the ceramic materials of monoclinic BAS and monoclinic SAS. Attempts to reduce the minimum ceramic process temperature for monoclinic BAS, SAS, or BAS+SAS solid solution ceramic material firing temperatures with the use of sintering aids has previously destroyed the mechanical and dielectric properties of the final ceramic material.
Dielectric ceramics, electronic packaging, and structural ceramics are possible applications for BAS, SAS, and BAS+SAS.
The sol-gel process is a well-known technique which uses the hydrolysis of a metal-organic compound to form a sol in the preparation of metal oxides. Clusters of small particles of colloidal metal oxides in the sol gather together. As the size of these clusters increases, the clusters become sufficiently immobile to produce a viscous liquid, which further forms into a solid colloidal gel structure. The sol-gel technique has been applied to the preparation of both single component oxide glasses and multi-component oxide glasses.
In view of the foregoing, a method of making monoclinic celsian at lower temperatures for shorter heating times has been desired. Additionally, increased control of the quantity and purity has been desired for the composition of monoclinic celsian.